Add cost_dollars field to AnswerResponse and StreamAnswerResponse

[Feel-ix-343]

Add cost_dollars field to AnswerResponse and StreamAnswerResponse

Summary

Fixes #134 - The API returns costDollars in answer responses, but the SDK was not exposing this field.

This PR adds the cost_dollars field to:

• AnswerResponse dataclass
• StreamChunk dataclass (for streaming responses)

The implementation follows the existing pattern used by SearchResponse for handling cost_dollars, reusing the existing parse_cost_dollars() helper function.

Review & Testing Checklist for Human

• Verify API response structure: Confirm that the /answer endpoint actually returns costDollars with the expected structure (total, search, contents breakdown). The CostDollars type was designed for search responses - verify it matches answer responses.
• Test streaming responses: Verify that costDollars is included in streaming chunks (typically the final chunk). I couldn't test this against the live API.
• Test with live API: Run a simple test like:

  from exa_py import Exa
  exa = Exa(api_key="...")
  response = exa.answer("What is Python?")
  print(response.cost_dollars)  # Should show cost breakdown

Notes

• All 109 unit tests pass
• The implementation mirrors how SearchResponse handles cost_dollars
• Link to Devin run: https://app.devin.ai/sessions/6523d6fb5b9d42368aa4b1bbe31f6f19
• Requested by: Felix Zeller (felix@exa.ai) / @Feel-ix-343

[devin-ai-integration]

Original prompt from Felix

fix this https://github.com/exa-labs/exa-py/issues/134

[devin-ai-integration]

🤖 Devin AI Engineer

I'll be helping with this pull request! Here's what you should know:

✅ I will automatically:

• Address comments on this PR that start with 'DevinAI' or '@devin'.
• Look at CI failures and help fix them

Note: I can only respond to comments from users who have write access to this repository.

⚙️ Control Options:

• Disable automatic comment and CI monitoring